Currently, light-emitting elements utilizing the electro luminescence (Electro Luminescence: EL) of organic materials gain attention as thin-type light-emitting materials.
A so-called organic electro luminescence element (hereinafter also referred to as an organic EL element) is a thin film-type, completely solid element capable of emitting light at a low voltage of about several volts to several ten volts, and has many excellent characteristics such as a high luminance, a high light emission efficiency, a thin type and a light weight. Therefore, organic EL elements gain attentions as surface light-emitting bodies for backlights for various displays, display boards such as signboards and emergency lights, and illumination light sources.
Such organic EL elements each has a constitution in which a light-emitting layer formed of an organic material is disposed between two electrodes, and the emitted light generated in the light-emitting layer goes through the electrodes and is extracted outside. Therefore, at least one of the two electrodes is constituted as a transparent electrode, and the emitted light is extracted from the side of the transparent electrode.
On the other hand, demands for substantially transparent organic EL elements capable of emitting light from two sides have been increased in view of high design property and the like. Furthermore, organic EL elements capable of emitting light from two sides having flexibility and a light weight have been required.
It was a conventional problem that, in the case when the upper side electrode that is on the position far from a substrate of an organic EL element among two electrodes on the substrate is comprised by a transparent material and hollow sealing is conducted, reflection occurs at the interface between the electrode and the sealed space, and the side of the lower electrode emits light more strongly, and the viewing angle dependence of chromaticity is so high that the shade differs depending on the angle of observation.
Furthermore, there was also a problem that the freeness of the design of the light emission states of the two sides is not high, since the light emission states of the substrate side and the sealing side are restricted due to the difference in the materials and layer thicknesses of the two electrodes on the upper part and lower part, the difference in the materials of the substrate and the sealing material, and the like.
Furthermore, in the case of a flexible substrate, when the organic EL element has a formed of a curved surface, regions having different viewing angles are generated in the same element panel surface even seen from the same place. Therefore, the above-mentioned viewing angle dependence of chromaticity becomes more significant, and thus a light emitting property in accordance with the Lambert distribution is required.
The above-mentioned viewing angle dependency is improved to some extent by using a light extraction sheet having a light scattering property. However, use of a light extraction sheet is not necessarily preferable in view of transparency, and it is preferable that the element itself has a light emitting property in accordance with the Lambert distribution.
As an organic EL element capable of emitting light from two sides, Patent Literature 1 has attained light emission from two sides by attaching two organic EL elements formed on transparent substrates so that the transparent substrates are positioned on the outer sides with each other, but the elements themselves are not transparent since the emitted lights are extracted from the sides of the transparent substrates. Furthermore, the layer thickness is thick due to the attaching, and thus there are problems in view of cost and producibility.
An object of Patent Literature 2 is to obtain an image display having even color tones of the emission of light toward the upper surface and the emission of light toward the lower surface and a high quality, by adjusting the layer thickness of a transparent electroconductive film disposed on the side of a cathode and the layer thickness of the cathode. However, since the element has an airspace inside, reflection occurs at the interface of the above-mentioned electrode and the sealed space, and thus there are problems that the side of the lower electrode emits light more strongly and thus even light emission from the two sides is impossible as mentioned above, and that the above-mentioned viewing angle dependence of chromaticity is also significant.
Patent Literature 3 discloses about a two-sided light emission-type organic EL element that can radiate emitted lights having different light properties from the two sides of the element, but does not explain in detail about the substrate, electrodes and sealing material. Furthermore, since the element has an airspace inside, and a wavelength converting material, scattering particles and the like are added to one of the electrodes, it is considered that the transparency is low, and the viewing angle dependence of chromaticity is significant.
Accordingly, under the current situation, a two-sided light emission-type organic electroluminescence element that is transparent, has flexibility, has a small viewing angle dependence of chromaticity, and can easily adjust the light-emission balance of the two sides has not been obtained.